This is 100% normal. The voltage you are viewing is the output from the alternator, not the actual voltage of the battery. A fully charged car battery only has a voltage of 12.6v and battery with only 50% capacity will be around 12.2v (These are the voltages of the battery with no load.)

The output voltage of the alternator will vary from around 13.5v to around 14.7v depending on the current charge of the battery, load on the electrical system, and the RPM's of the engine.

When the car is running you have two sources of electricity, the alternator and the battery. The alternator almost exclusively provides all of the electrical power while the car is running except when the load is more than the alternator is currently generating. When load exceeds the supply provided by the alternator, the battery (power sink) will provides the extra amps.

The alternator provides power at a pretty steady rate, but when a surge of power is required for just an instant, this power is provided by the battery This happens pretty rarely, usually only when an electrical motor first turns on (like your power windows, or radiator fans)

A stereo amplifier is a different kind of animal and does not draw power at a steady rate. If you have your stereo cranked, every beat of the drum will draw significant amounts of power for just an instant and a bad battery or one that doesn't have enough capacity won't be able to provide enough power and the result is a voltage drop. If you have a high power stereo system you will need a high capacity battery (or multiple batteries wired in parallel) and/or an alternator that provides more power.

 

By high capacity battery do you mean one with the most CCA and highest reserve power?
For my car AC-Delco offers two batteries one has more power in the above areas and has a longer warranty. Cost difference is like $95.00 vs. $150.

 

if your alternator is only pushing 12v then there's something wrong. Every time it shows 12 volts... that means that your battery is now supplying voltage (or at least its contributing) to power the car. In essence, if you disconnect the battery while the car is running, the car would shut off which it shouldn't if you have a healthy alternator.
So whats going on is that at idle, you are slowly pulling juice from the battery but any time your not at idle and driving, the alternator pumps juice to everything and charges the battery back up only for it to drain again at idle. Problem is that once your battery is on its last leg, your car will die while at idle which it shouldn't.
Take a voltmeter to the battery and verify the readings. If your getting 12 volts at idle and the voltage climbs as you give gas..... I would replace the alternator cause a good one should read no less than 13 volts at idle with all your accessories on.

 

If it's an auto trans it's normal!!! I'm betting all those that are saying theirs doesn't drop below X have manual trans and let it idle in neutral at traffic lights. If they have autos and it doesn't drop at traffic lights with power being consumed from major items (radio, A/C) then their cars are "special" and should be used as experimental test subjects cause they found the all important self sustainable power source everyones been looking for!!!!

 

Ill check the alternator today...I do have an amplifier and a subwoofer and its a 5 speed. So if i do the test and the car still runs with the battery disconnected and the alt still shows 12 volts does that mean its in is way out?

 

I have an 05 w/A4. With everything on at night sitting in traffic and 105° outside mine will sometimes go into the 11s. I dont like seeing that and posted about here too. As soon as I start moving it goes back to 13v. It starts no problem and is over 14v every time so I agree that its normal. I have 2 aftermarket amps going too.

 

yeah, the amp and sub will make it go down a little....

 

What you guys are saying isn't always true. I have an '07 Tahoe that has a feature that shuts down the alternator to improve gas mileage. when in shut down mode the voltage shows about 11 to 11.5 volts. when the alternator comes back on line the voltage goes up to 13.5 to 14.5 volts. this happens several times an hour and can be quite disconcerting unless you know whats happening.

 

Update, i disconnected the battery while the car was running and it didn't cut off, the tensioner seems to be ok and i checked the optima site for a yellow top..couldn't find one, only red...i email them to see if they even make a yellow top for 05 vettes...about to check battery water level...

 

Same car and situation here.

 

mine dips to 13 constant and when the sub really kicks the amp will suck it down to 11 for a second, but i have no problems

 

My C6 does that.
My ZO6 C5 does the same thing.

My C4 does it too. Before changing batteries the system voltage indicated by the digital dash was usually around 13.8~13.9.
With a new battery it now reads 13.5 volts with the AC, radio, headlights and fog lights on when cruising at +/- 1800-2000 RPM.
Voltage decays to around 12.8 when stopped in traffic.

Besides the C4, C5 and the C6 I have an LS1 Z28 and the volt meter acts the same as yours and mine.

IMHO you don't have any problem worth being concerned about.

BTW, It is N_O_T recommended to disconnect the battery from the system while the engine is running.
You could get excessive voltage, voltage spikes or AC ripple from the output of the alternator with no battery to absorb them and no battery supplying the field voltage for the alternator.

On the older BC cars (before computers) this wasn't a problem.
Doing this now has the chance of costing you super big bucks to replace one or more of the computers on your car.

Just because you got by with it this time doesn't mean it won't fry a lot of electronic components next time.

No reflection on you because you just didn't know this.

Glad your electrical system is functioning normally.

 

also in an auto not only are you rpms lower but I think the brake lights consume some decent power too

 

Good point. I never thought about that.
Jeff

 

How was your water level when you checked it?

Mike

 

Yeah man...don't do that again!!! That test went out the window 30 years ago.

One more thing...

You can't properly analyze any portion of your charging system unless you have first fully charged and LOAD TESTED your battery. Wish I had a dollar for every guy who has thrown an alternator or battery in their car because they made a bad assumption.

 

Say what? So, manual trans cars don't have radio, AC, etc? From another post I guess if you have a manual trans you don't keep the brakes on while you're stopped either.

Or not.

As others have said, get the battery load tested, then get the charging system tested once the battery is verified to be good. You can get both tested for free at most auto parts stores.

 

OK - I checked my battery today. It was very low on electrolyte.
I added and added distilled water. It took a lot.
The plates were not exposed when I pulled the caps. My car is only 9 months old now.

Mike

 

Looks normal to me.

 

Steve R - I think the auto/man issue is idle speed -- my A4 hot idle IN gear is only about 500rpm -- that combined with fans running is the only time my voltage has dropped below 13.